It is well known that certain external analgesic products containing more than 3 percent to 11 percent camphor and 1.25 to 16 percent menthol, either singly or in combination, cause irritation or mild inflammation of the skin for the purpose of relieving pain in muscles, joints, or viscera distal to the site of application by stimulating depressing cutaneous sensory receptors.
Although camphor and menthol have been used in external analgesic products, their crystals have not been used in lotions because the crystals are immiscible with the solutions formed by the other ingredients used.